Xylans are polysaccharides present in the cell walls of all plants. They comprise the major non-cellulosic component of secondary walls of angiosperms. Hence, xylans are quantitatively second only to cellulose in biomass on earth. Xylans are valuable components of human and animal nutrition, constituting a major component of dietary fiber in cereals (1). The amount of xylan as well as its chemical composition is an important element of bread-making, influencing dough yield, flavor and shelf life (2). Xylan is of particular interest for the improvement of feedstocks for the generation of cellulosic biofuels, a currently expensive and inefficient process (3). Xylan inhibits access of cellulases to cellulose, and xylan is an additional substrate for cross-linking to lignin.
Xylans play an important structural role in plant cell walls, presumably through interactions with cellulose microfibrils and other components of the wall (4, 5). While the organization of these various components in the cell wall is not well understood, the increased cross-linking apparently contributes to the recalcitrance of the cell wall (6, 7). Xylans are structurally diverse between taxonomic groups (4). The most basal form of xylan is a main chain that lacks substitutions as seen in the green algae Caulerpa that has β-1,3-D-xylose in place of cellulose, and the red seaweeds Palmariales and Nemaliales that have a mixed linkage β-(1,3-1,4)-D-xylose backbone (8). Xylans of embryophytes have a β-1,4-linked xylose backbone. Xylans found in dicots are mostly restricted to the secondary cell walls, and hence a main component of wood. Dicot xylans are commonly substituted with α-(1→2)-linked glucuronosyl and 4-O-methyl glucuronosyl residues (1). Xylan in birch, spruce, and Arabidopsis have been found to contain the reducing end oligosaccharide β-D-Xylp-(1→4)-β-D-Xylp-(1→3)-α-D-GalpA-(1→4) D-Xylp (9-11) which, interestingly, has not been found in the xylan of grasses. Commelinid monocot xylans are unique from dicots and other monocots. This group, which includes the grasses, contains xylan as the main non-cellulosic component in both the primary and secondary cell walls. These xylans have very little glucuronic acid, but are mostly substituted with α-1,2 and α-1,3 arabinosyl residues. A unique feature of grass xylans is the ferulate and coumarate esters attached to some of the α-1,3 arabinosyl residues. These ferulate esters mediate intra- and intermolecular cross-linking, possibly increasing the strength of the cell wall (7). Another feature that makes grass xylan unique is the β-1,2 linked xylose residues found on these feruloyl-arabinofuranose substitutions (12-16).
During the past six years, there has been significant progress made toward identifying the glycosyltransferases (GTs) that synthesize xylan. The irregular xylem (irx) mutants, named for their collapsed xylem vessels due to their secondary cell wall deficiency, have been instrumental in elucidating the mechanisms of xylan biosynthesis in Arabidopsis thaliana (Arabidopsis). IRX9/IRX9L and IRX14/IRX14L from the GT43 family, and IRX10/IRX10L from GT47 are thought to be responsible for elongation of the xylan backbone (17-21). We have recently identified and characterized a loss of function mutation in the rice IRX10 homolog and shown that OsIRX10 is important for xylan deposition in vascular tissues of the stem (22). IRX7 (FRA8)/IRX7L (F8H) (from GT47), IRX8 (from GT8), and PARVUS (from GT8) may be responsible for synthesizing the oligosaccharide found at the reducing end of some dicot and conifer xylans (4, 18, 23, 24). GUX1 and GUX2 (from GT8) are thought to be responsible for adding both glucuronic acid and 4-O-methylglucuronic acid branches to xylan in Arabidopsis (25, 26). Recently, rice and wheat GT61 family genes were found to be responsible for α-(1,3)-arabinosyl substitution on xylan (27). Thus far, the enzymes that add β-(1,2)-xylose to xylan have not been identified.